Cook the pasta, reserving 1/4 cup of cooking liquid.
When done, drain lightly, and place in a warm serving bowl, or directly into pan with the melted butter, immediately adding watercress, cheese and several grinds of the pepper mill.
If you prefer the sauce moister, or it seems too dry, add a tablespoon of the reserved liquid at a time till desired moistness is reached.
Mix well and serve immediately onto pre-warmed plates.